Is your tummy rumbling today? Are you watching the clock already waiting for night to fall when you can scoff 500 calories and tick off another successful day of fasting on the 5:2 diet, like thousands of others following the most successful diet plan devised this century? Well, get ready to rumble some more. While we've all been systematically starving ourselves twice a week following the Fast Diet, a newer take on fasting has been under trial in the US.

And - grave news, hungry readers - this nutritional regime, known as the Fasting Mimicking Diet (FMD), developed by academics at the University of Southern California (USC), demands we starve for five days straight for maximum effect.

What might make this new fasting diet do-able, is that according to its creators, most of us would only have to do this four times a year to reduce our risk of heart disease and cancer as well as boost our immune systems, lose weight and look younger, too. No need to join the slightly bonkers world of Calorie Restrictors (who consume only 75 per cent of recommended calories a day, and who practise savoury meditation (smelling and thinking about food rather than eating it). Or the breatharians who claim to live by sunlight alone.

One of the scientists behind this revolutionary new way of fasting is Prof Valter Longo, Professor in Gerontology and Biological Science at the USC, and director of its Longevity Institute. Experience may have taught us that typical uber-fasting fanatics tend to be ghastly old geezers who look like they are made of biltong. But the 47-year-old professor appears to be part Italian midfielder, part Bambi; all in all a rather marvellous walking advertisement for the FMD. And best of all, his research has the rigour of science behind it.


"Strict fasting is hard for people to stick to, and it can also be dangerous," Prof Longo tells me. "So we developed a complex diet that triggers the same effects in the body." In his study, reported this week in the journal Cell Metabolism, old mice placed on cycles of a four-day low-calorie diet had reduced visceral belly fat, and increased numbers of progenitor and stem cells in several organs - including the brain, where neural regeneration was boosted, as was memory. Meanwhile, a trial of 19 humans placed on the FMD once a month for five days showed decreased risk factors and biomarkers for aging, diabetes, cancer and heart disease with no major adverse side effects.

What this means in practice for you and me is a diet reduced to between 34 to 54 per cent of normal calorie intake with a specific composition of proteins, carbohydrates, fats and micronutrients which could unlock the key to anti-ageing and maybe extend your life. It breaks down like this: on day one, the overall diet adds up to 1,090 calories - which must be consumed as 10 per cent protein, 56 per cent fat and 34 per cent carbohydrate. Days two to five contain 725 calories each, split into portions of 9 per cent protein, 44 per cent fat and 47 per cent carbohydrate.

It sounds all so complicated. So why is this new diet so much better than the 5:2, I ask Prof Longo (catching him during a lunch break, which he says rarely consists of actual, you know, lunch). "I don't like to say better or worse," he protests. "But the composition is important; we worked hard to find the exact amounts, so that you don't feel like you are fasting, but your body gets the same effects. The time (five-day stretches) is important because the body needs to do this for three or four days before reprogramming itself, and the process of regeneration and rejuvenation begins."

He adds of the 5:2, first outlined by BBC medic Dr Michael Mosley who co-authored the bestselling The Fast Diet book with journalist Mimi Spencer: "The 5:2 - it is not bad, but you need to do it every week. With the Fasting Mimicking Diet, you do it every three or four months, so that's less than half the time."

He also thinks there are two specific problems with 5:2. "We have seen that the high protein intake associated with 5:2 and the short duration of the fast (ie, 24 hours) seems to block the regenerative effect you get from fasting." Plus, the brain gets confused when you ask it to change behaviour often; fasting and then not fasting, especially if you swap the days around a lot may affect our Circadian rhythms, says Prof Longo. So you could end up struggling to sleep at night on the 5:2. He is most certainly not a fan of extreme fasting: "Water-only fasting should only be done in a specialised clinic and can increase the incidence of gallstones in women at risk if done improperly."

Petronella Ravenshear, a nutritional therapist in London, agrees that Prof Longo's way may be slightly easier on the body than other types of fasting. She says: "Fasting, or restricting calories, to improve health and increase lifespan, is not a new idea. But this fasting mimicking diet is less of a stressor on the body than complete fasting. It supplies most of the carbohydrates in the form of vegetables which are packed with phytonutrients and minerals and positively good for us, rather than grain-derived carbohydrates which don't supply much except sugar. One of the hormones this diet reduces is insulin-like growth factor (IGF-1), high levels of which are correlated with increased risk of cancer."

She adds: "What would be even better would be to encourage people on the non-fasting days to eat two to three times a day rather than going back to their old ways of grazing and snacking. Decreasing meal frequency is pretty painless and by reducing meal frequency we naturally reduce inflammation (the hallmark of modern chronic disease) in our bodies."

However, even Prof Longo says he still wants to see his findings confirmed from larger human trials (which are under way). His aim is not to see people lose weight necessarily, but to help end the global medical culture which revolves around pill-popping.

"The system is prehistoric, it's expensive, and it's making us all broke," he says of the curative medical tradition. Ideally, he hopes those following the FMD a couple of times a year will reduce the number of biomarkers for cancer sufficiently to prevent the disease.

So when can we start? Currently, Prof Longo warns against improvisation and says to consult a doctor or dietician before trying the FMD. He is currently writing a definitive guide to the FMD. L-Nutra will produce a five-day nutrition pack, containing soups, drinks and bars, called ProLon. All profits from the book and ProLon will be donated to charity, he says. While new diets have created multi-million empires for some, it seems this is not a money-making project.

Nor is it an attempt to rewrite global nutrition. Prof Longo says: "We don't want to change what people eat normally; we are not advertising restriction. In general, I would say, if you follow the FMD twice a year, as I do, and combine it with general nutritional advice like that, it will make a tremendous difference to your longevity."

The Fake Fast Diet

Day One

FMD permitted calories: 1,090 max
Breakfast: Black or Green Tea; one boiled egg (78 calories) + one slice whole wheat toast (68 calories)
Lunch: black coffee or tea; small green salad with avocado, dressed with olive oil (300 calories)
Snack: two almonds (28 calories)
Dinner: Large helping of mixed green vegetable soup with borlotti beans, and slice of whole wheat bread (616 calories)

Day Two

FMD permitted calories: 725 calories max
Breakfast: Black or green tea; one poached egg with a grilled tomato (100 calories)
Lunch: Miso soup (21 cals)
Snack: 7 walnut halves (90 cals)
Dinner: Vegetable chilli with kidney beans and two tsp sour cream (514 cals)

Day Three:

FMD permitted calories: 725 calories max
Breakfast: Black or Green tea: one slice whole wheat toast with two tsp of cashew butter (150 calories)
Lunch: Espresso; Smoked Salmon (100g) with watercress (200 cals)
Snack: Blueberries (100g) (57 cals)
Dinner: Large portion vegetable soup (318 cals)

Day Four:

FMD permitted calories: 725 calories max
Breakfast: Black or Green tea; half an avocado on one slice of whole wheat toast (220 cals)
Lunch: Espresso; 250ml glass of almond milk (60 cals)
Snack: two squares 70 per cent dark chocolate (110 cals)
Dinner: Large green salad with 100g prawns, dressed with olive oil and lemon juice (335 cals)

Day Five

FMD permitted calories: 725 calories max
Breakfast: black or green tea; two boiled eggs (156 cals)
Lunch: Half an avocado on toast; miso soup (210 cals)
Snack: An apple (60 cals)
Dinner: Large portion of vegetable soup with 10g toasted pine nuts (299 cals)